This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Primary Objectives: To assess the effects of chronic CoQ treatment in slowing the functional decline of subjects with HD, as determined by the Unified Huntington's Disease Rating Scale (UHDRS). To assess the blood levels of CoQ at the studied dosages. Secondary Objective: To examine the long term safety and tolerability of CoQ in the HD population. To compare subjects treated with CoQ with those treated with placebo with regard to changes from Baseline to Month 60 in other components of the UHDRS.